Current and voltage controlled oscillators (ICO and VCO) are important components in the structures of transmitters and receivers. When applications in portable wireless communications systems are concerned, the main requirements for VCO/ICOs are: an operational frequency range of 1 to 20 GHz, a very low phase noise, and the lowest possible operating voltage and power consumption. Depending on the structure, a communications device may comprise several VCO/ICOs needed for different purposes, e.g. frequency conversion, synthetization, modulation, etc. Their performance affects strongly the performance of the entire communications unit. However, the demand to implement these oscillators for silicon technologies faces several problems.
During the last few years several research projects have focused on finding optimal solutions. Two types of oscillators are mainly used as the cores of VCO/ICOs: sinusoidal oscillators and relaxation oscillators. Sinusoidal oscillators usually produce the best parameters as far as high frequency and low phase noise are concerned, but they can be easily implemented mostly on GaAS technologies only. A transition to bipolar, CMOS or BiCMOS technologies causes several problems mainly due to the highly conductive substrate. On the other hand, the speed of such available technologies is a challenge to researchers, as at present transient frequencies of 10 to 40 GHz are reached, which was previously considered to be a transient range possible to be covered only by materials based on GaAS. The speed of silicon-based technologies is sufficient enough for mobile communication in the frequency range of 1 to 20 GHz, used by most mobile stations and wireless LANs. An additional driving factor in the design of portable equipment has always been a high demand for as low an operating voltage as possible and a very low power consumption.
In oscillators of LC type, the active circuit components are kept out of the non-linear operation range, whereas in relaxation oscillators, the sinusoidal signal is the result of the incapability of the pulse circuit to switch fast enough at very high frequencies.
Due to the operation in the non-linear operation range, many high-energy spectral components occur in the output signal. A very `clean` spectrum can thus be obtained by developing oscillators of LC type. While the relaxation types only need a reference capacitance, the LC types require, in addition to a contour capacitance, also an inductance having a reasonably high Q factor. This causes technological difficulties.
Ring oscillators are a subclass of relaxation oscillators. A ring oscillator comprises several (typically at least three) differential delay stages in cascade, such as stages A1, A2, A3 and A4 in FIG. 1. In cascade, the noninverted output of a delay element of each stage A1 to A3 is connected to the non-inverting input of the next stage and the inverted output to the inverting input. The outputs of the last stage are connected to the inputs of the first stage in the same way as in the other stages or, as in FIG. 1, the inverted output of the last stage A4 is connected to the non-inverting input of the first stage A1 and the non-inverted output to the inverting input, respectively. An oscillation period of a ring oscillator is twice the number of delays (stages).
The delay stages of a ring oscillator usually are differential amplifiers implemented as a differential pair, as shown in FIG. 2. The collectors of NPN transistors Q1 and Q2 are connected via resistors Rc to one potential of an operating voltage source. The emitters of Q1 and Q2 are in turn connected via a current source 21 and a resistor Re to another operating voltage potential. The bases of Q1 and Q2 provide stage inputs and the collectors provide stage outputs. The differential stages are usually joined together via coupling buffers, such as emitter followers. Further, the outputs of a differential stage are applied to active mixers 11, 12 and 13 to be mixed together and with the outputs of the other stages so that a desired quadrature output is produced from the ring oscillator. Ring oscillators are described e.g. in the following publications:
1! B. Razavi and J. Sung, "A 6 GHz 60 mW BiCMOS Phase-Locked Loop", IEEE Journal of Solid-State Circuits, Vol. 29, No. 12, December 1994, pages 1560 to 1565; PA1 2! "High-speed voltage-controlled oscillator with quadrature outputs", Electronic Letters, 14th Feb. 1991, Vol. 27, No. 4, page 309. PA1 a first inductive component, via which a first main electrode of the first amplifier component is connected to a first operating voltage potential, PA1 a second inductive component, via which a first main electrode of the second amplifier component is connected to the first operating voltage potential, and PA1 a series connection of a third inductive component and a current source between interconnected second main electrodes of the first and second amplifier component and a second operating voltage potential.
Voltage and current controlled ring oscillators with quadrature outputs (90 degree phase difference between the outputs) are very important building blocks for many high-speed applications. Such applications are e.g. Phase-Locked Loops (PLL), Binary-Phase-Shift-Keyed (BPSK) demodulators, discriminators, clock recovery circuits for wireless and optic receivers.
The primary parameter to be reached is the maximum frequency. So far, nearly f.sub.MAX (the highest frequency set by the manufacturing process of transistors) has been reached by using oscillators of "lumped" resonator type, which are difficult to implement even by a manufacturing process of gallium-arsenic (GaAs) type. Solutions of other types, even if they are fast, have certain difficulties to reach close to the f.sub.MAX value of the manufacturing process.
Implementation of a voltage or current controlled ring oscillator requires an addition of a suitable control solution to the circuit. Frequency control usually changes either the DC biasing of certain components or alternatively the reference currents, which affect the timing of relaxation, and the frequency generated through that.
Other requirements of oscillators are e.g. low power consumption and low operating voltage, especially in portable electronic devices using battery power supplies.